Hi Moshe:
The voltage of the neutral with respect to the ground wire
is a function of its resistance and its current:
E = I * R
Maximum normal current is the rating of the fuse or circuit-
breaker protecting that particular circuit. For a 120-volt
circuit in the USA and Canada, the current would be either
15 or 20 amperes.
The resistance is a function of the wire cross-sectional
area and its length.
According to a licensed electrical engineer colleague, circuits
in the USA are designed for a maximum 6% voltage drop, preferably
3% voltage drop at the load. This means that the voltage drop
across the neutral from the load to the distribution transformer
is 3% maximum, 1.5% preferable.
So, the resistance is selected according to maximum voltage
drop at the circuit rating.
The answer to your question is that the neutral voltage varies
from zero to 1.5% nominal, 3% maximum, of the supply voltage
depending on load.
It further varies with the distance of the point of interest
from the point where the neutral is grounded. If the point of
interest is within a few feet of the point where the neutral is
grounded, then the neutral voltage will be a fraction of a
percent. If the point of interest is near the maximum distance
from the grounding point, and if the load is also at that point,
then the neutral voltage will approach 1.5% nominal to 3%
maximum.
Fault current is a function of time. Overcurrent devices will
operate according to their time-current curves. So, if you
know the fault current, then you can calculate the voltage at
any point along the neutral according to where the fault
current is introduced and where the neutral is grounded.
Fault current exists only for the period of time it takes to
operate the overcurrent device (I-T curve).
For a 120-volt system, you can expect about 1.8 volts drop
nominal, 3.6 volts maximum, assuming a single, maximum load
furthest from the point of grounding, and your measurement is
also at that point.
Best regards,
Rich
-------------------------------------------------------------
Richard Nute Product Safety Engineer
Hewlett-Packard Company Product Regulations Group
AiO Division Tel : +1 858 655 3329
16399 West Bernardo Drive FAX : +1 858 655 4979
San Diego, California 92127 e-mail: [email protected]
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